Soda Stream, an Israeli company, located it's factory in an Israeli occupied settlement, which many in the international community see as illegal. Pro-Palestinian activists oppose the factory for that reason.
What does Scarlett Johnasson have to do with the controversy? The actress is a spokesperson for Oxam, but when she starred in the Soda Stream commercial, Oxfam criticized her. That's because Oxfam, like much of the international community, oppose Soda Stream's location on the occupied West Bank. Oxfam said that Soda Stream furthered "the ongoing poverty and denial of rights of the Palestinian communities that we work to support.”
Johansson resigned her spokesperson position with Oxfam and defended Soda Stream saying that she supports "building a bridge to peace between Israel and Palestine, supporting neighbors working alongside each other..."
BuzzFeed has an excellent story about the controversy here.
This story might help students better understand the conflict between Palestinians and Israelis.